Optically semitransmissive film, photomask blank and photomask, and method for designing optically semitransmissive film

ABSTRACT

The present invention provides an optically semitransmissive film that has a near-zero phase shift, has a desired transmissivity, and is relatively thin; a novel phase-shift mask that uses the optically semitransmissive film; a photomask blank that can [be used to] manufacture the phase-shift mask; and a method for designing the optically semitransmissive film. The film is formed on a translucent substrate and transmits a portion of light having a desired wavelength λ, wherein the film has at least one phase-difference reduction layer that fulfills the following functions. Specifically, the phase-difference reduction layer is a layer that has a refractive index n and a thickness d that satisfy the expression 0&lt;d≦λ(2(n−1)), and is a layer in which the value Δθ (hereinafter referred to as phase difference Δθ (units: degrees)) obtained by subtracting the phase of light (hereinafter referred to as layer-referenced light) in the absence of a layer from the phase of light (hereinafter referred to as layer-transmitted light) transmitted through a layer is less than the value Δθ 0 =(360/λ)×(n−1)×d (hereinafter referred to as the phase difference Δθ 0  (units: degrees)) calculated based on the difference in the optical distance between the layer-transmitted light and the layer-referenced light.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Division of application Ser. No. 11/629,210 filed Jan. 3,2007, which is a National Phase of Application No. PCT/JP2005/010713filed Jun. 10, 2005 which claims priority to Japanese Patent Application2004-178895 A, filed Jun. 16, 2004 and Japanese Patent Application2005-137171 A, filed May 10, 2005. The disclosure of the priorapplications is hereby incorporated by reference herein in its entirety.

TECHNICAL FIELD

The present invention relates to an optically semitransmissive filmwhich has a near-zero phase difference, a photomask blank and photomaskprovided with the optically semitransmissive film, and a method fordesigning the optically semitransmissive film.

BACKGROUND ART

An optically semitransmissive film in a halftone phase-shift mask is anexample of an optically semitransmissive film that partially transmitsexposure light and in which the amount of phase shift is controlled.

A phase-shift mask is a photomask used in one type of super-resolutionlithography in which photomasks are used in the lithographic step, whichis one step in LSI manufacturing. Pattern contrast is improved by aninterference effect (hereinafter referred to as the “phase-shifteffect”) that is brought about by changing by 180° the phase of aportion of the light transmitted through the photomask. A halftonephase-shift mask is one type of phase-shift mask, and exposure lightthat passes through apertures with a prescribed dimension has anintensity distribution whose spread is greater than the dimensions ofthe apertures due to diffraction. The optical intensity [of light] *1 inthe spread-out portion is cancelled out by the interference effect inwhich [the light] passes through the adjacent optically semitransmissiveareas and interferes with light (interference light) whose phase isshifted by 180°, and the contrast of the boundary area of the two isincreased.

Such halftone phase-shift masks are relatively easy to use in themanufacturing process, and are therefore the mainstream phase-shiftmasks currently used in KrF and ArF lithography. A single-layer thinfilm composed of Mo—Si—N is generally used as the material of thephase-shift film in a halftone phase-shift mask, the transmissivity is 5to 15%, and the film thickness is about 60 nm to 100 nm.

A novel phase-shift mask is proposed in Patent Document 1. The mask isused to transfer a pattern that is compatible with a smaller LSI. Thisphase-shift mask transmits 15% or less of the exposure light and has alight-blocking film (optically semitransmissive film) that has a phaseshift of (−30+360×m) degrees or more and (30+360×m) degrees or less(wherein m is an integer) between the light transmission areas. Alsodisclosed is a phase-shift mask provided with a phase shifter forgenerating a phase difference of (150+360×m) degrees or more and(210+360×m) or less (wherein m is an integer) in the apertures disposedin the light-blocking film.

-   Patent Document 1: Japanese Laid-open Patent Application No.    2003-21891

DISCLOSURE OF THE INVENTION Problems that the Invention is to Solve

In a conventional phase-shift mask, the phase difference Δθ₀ between thephase θ of light that passes through a phase-shift film and the phase θ′of light that passes through the apertures is proportional to thedifference ΔL in the optical distance between the two transmitted lightbeams. For example, the following equation is used in the case of asingle-layer halftone phase-shift mask, which is currently themainstream phase-shift mask as described above.Δθ₀(360/λ)×ΔL=(2π/λ)(nd−d)[rad]  (Eq. 1)In the equation, the refractive index of the atmosphere in which themask is placed is 1, and n and d are the refractive index and physicalfilm thickness, respectively, of the single-layer film materialcomprising the phase-shift region. Therefore, for a phase-shift mask ina conventional single-layer halftone phase-shift mask, the designguideline is to set n and d in accordance with the exposure wavelength λso that Δθ=π, i.e., the following equation holds true.d=λ/(2(n−1))|  (Eq. 2)Also, in the case that the phase-shift region is composed of a pluralityof material layers, the following equation holds true if the refractiveindex of each of the materials is n1, n2, . . . , nj.Δθ₀=(2π/λ)(n1+n2+ . . . +nj)d−d)[rad]  (Eq. 3)Eqs. 1 and 3 indicate that a phase-shift difference is being produced bylight traveling through the phase-shift region.

However, when an optically semitransmissive film used as the phase-shiftmask proposed in Patent Document 1 is designed with the same designphilosophy as described above, i.e., with a phase difference of(−30+360×m) degrees or more and (30+360×m) degrees or less (wherein m isan integer), the film thickness d must be reduced in the case that m=0,and a higher transmissivity results. In the particular case ofattempting to achieve a phase difference of zero, the film thickness dmust be made infinitely close to zero. When m=1, the film thicknessbecomes considerable, e.g., 100 nm or more, the transmissivity isreduced, and the dimensional accuracy is reduced during etching that iscarried out in order to form a master pattern for a phase-shift mask.There is therefore a need for a novel design philosophy in order todesign an optically semitransmissive film whose phase shift is close tozero, such as the film disclosed in Patent Document 1.

The present invention was contrived in view of the aforementionedproblems, and an object is to provide an optically semitransmissive filmthat has a nearly zero phase shift, has a desired transmissivity, and isrelatively thin, and to provide a method for designing the opticallysemitransmissive film.

Another object of the present invention is to provide a photomask blankthat can be used to manufacture a novel phase-shift mask such as thatdescribed in Patent Document 1, for example, by providing an opticallysemitransmissive film that has a nearly zero phase shift, has a desiredtransmissivity, and is relatively thin.

Another object of the present invention is to provide a novelphase-shift mask such as that described in Patent Document 1, forexample, by using a photomask blank provided with an opticallysemitransmissive film that has a nearly zero phase shift, has a desiredtransmissivity, and is relatively thin.

Means of Solving the Problems

In order to solve the aforementioned problems, a first aspect of thepresent invention provides an optically semitransmissive film which isformed on a translucent substrate and which transmits a portion of lighthaving a desired wavelength λ, wherein

the optically semitransmissive film has a refractive index n and athickness d that satisfy the expression 0<d≦λ(2(n−1)); and

[the film] has at least one phase-difference reduction layer in whichthe value Δθ (hereinafter referred to as phase difference Δθ (units:degrees)) obtained by subtracting the phase of light (hereinafterreferred to as layer-referenced light) in the absence of a layer fromthe phase of light (hereinafter referred to as layer-transmitted light)transmitted through a layer is less than the value Δθ₀=(360/λ)×(n−1)×d(hereinafter referred to as the phase difference Δθ₀ (units: degrees))calculated based on the difference in the optical distance between thelayer-transmitted light and the layer-referenced light.

The second aspect of the present invention provides the opticallysemitransmissive film according to the first aspect wherein thephase-difference reduction layer is used as one medium; another mediumis used which is selected from the translucent substrate, another layer,or atmosphere adjacent to the phase-difference reduction layer; and adiscontinuous phase change is generated in the boundary between thephase-difference reduction layer and the other medium adjacent to thephase-difference reduction layer, in contrast to the continuousintra-medium phase change in the two media, when light has beentransmitted to the optically semitransmissive film having thephase-difference reduction layer, whereby the phase difference Δθ ismade less than the phase difference Δθ₀.

The third aspect of the present invention provides the opticallysemitransmissive film according to the first or second aspect whereinthe difference between the phase difference Δθ and the phase differenceΔθ₀ is 10 degrees or more.

The fourth aspect of the present invention provides the opticallysemitransmissive film according to any of the first to third aspectswherein the phase difference Δθ is a negative value.

The fifth aspect of the present invention provides the opticallysemitransmissive film according to any of the first to fourth aspectswherein the extinction coefficient k and refractive index n of thematerial of the phase-difference reduction layer satisfy the expressionk≧n.

The sixth aspect of the present invention provides the opticallysemitransmissive film according to the fifth aspect wherein theextinction coefficient k of the material of the phase-differencereduction layer is 1.5 or less.

The seventh aspect of the present invention provides an opticallysemitransmissive film which is formed on a transparent substrate andwhich transmits a portion of desired exposure light, wherein

the optically semitransmissive film has a refractive index n and athickness d that satisfy the expression 0<d≦λ/(2(n−1)); and

[the film] comprises a multilayer structure having at least onephase-difference reduction layer in which the phase difference Δθ isnegative, and at least one other layer that is different than thephase-difference reduction layer.

The eighth aspect of the present invention provides an opticallysemitransmissive film which is formed on a translucent substrate andwhich transmits a portion of desired exposure light, wherein theoptically semitransmissive film has a refractive index n and a thicknessd that satisfy the expression 0<d≦λ/(2(n−1)); [the film] comprises amultilayer structure having at least one phase-difference reductionlayer in which the phase difference Δθ is negative, and at least onelayer in which the phase difference Δθ is positive; and the phasedifference of the entire optically semitransmissive film is a desiredphase difference in which the negative phase difference and positivephase difference are canceled out.

The ninth aspect of the present invention provides the opticallysemitransmissive film according to the seventh or eighth aspect whereinthe phase-difference reduction layer is used as one medium; anothermedium is used which is selected from the translucent substrate, anotherlayer, or atmosphere adjacent to the phase-difference reduction layer;and a discontinuous phase change is generated in the boundary betweenthe phase-difference reduction layer and the other medium adjacent tothe phase-difference reduction layer, in contrast to the continuousintra-medium phase change in the two media, when the exposure light hasbeen transmitted to the optically semitransmissive film having thephase-difference reduction layer, whereby the phase difference Δθ ismade negative.

The tenth aspect of the present invention provides the semitransmissivefilm according to any of seventh to ninth aspects, further comprising anantireflective layer as a layer in which the phase difference ispositive.

The eleventh aspect of the present invention provides thesemitransmissive film according to the tenth aspect wherein thewavelength of the light is a wavelength selected form 150 to 250 nm, andthe reflectivity of the optically semitransmissive film is 30% or lesswith respect to the light having the wavelength.

The twelfth aspect of the present invention provides thesemitransmissive film according to any of the seventh to eleventhaspects wherein the extinction coefficient k and refractive index n ofthe material of the phase-difference reduction layer satisfy theexpression k≧n.

The thirteenth aspect of the present invention provides thesemitransmissive film according to the twelfth aspect wherein theextinction coefficient k of the material of the phase-differencereduction layer is 1.5 or less.

The fourteenth aspect of the present invention provides thesemitransmissive film according to any of the seventh to thirteenthaspects wherein the extinction coefficient k and refractive index n ofthe material of the layer having the positive phase difference satisfythe expression k<n.

The fifteenth aspect of the present invention provides thesemitransmissive film according to any of the first to fourteenthaspects wherein the phase difference Δθ of the entire opticallysemitransmissive film is a desired phased difference selected from arange of −30° to +30°.

The sixteenth aspect of the present invention provides thesemitransmissive film according to any of the first to fifteenth aspectswherein the transmissivity of the optically semitransmissive film is 40%or less.

The seventeenth aspect of the present invention provides thesemitransmissive film according to any of the first to fifteenth aspectswherein the transmissivity of the optically semitransmissive film is 15%or less.

The eighteenth aspect of the present invention provides thesemitransmissive film according to any of the first to seventeenthaspects wherein the thickness of the optically semitransmissive film isin a range of 1 to 50 nm.

The nineteenth aspect of the present invention provides a photomaskblank for manufacturing a photomask having a mask pattern comprising anoptically semitransmissive film which is formed on a translucentsubstrate and which partially transmits desired exposure light, whereinthe optically semitransmissive film according any of the first toeighteenth aspects is used as the optically semitransmissive film.

The twentieth aspect of the present invention provides the photomaskblank according to the nineteenth aspect wherein a light-blocking filmis formed on the optically semitransmissive film.

The twenty-first aspect of the present invention provides the photomaskblank according to the twentieth aspect wherein the opticallysemitransmissive film comprises a material containing Si, or Mo and Si;and the light-blocking film comprises a material containing Cr.

The twenty-second aspect of the present invention provides a photomaskcomprising an optically semitransmissive area in which the opticallysemitransmissive film in the photomask blank according to any ofnineteenth to twenty-first aspects is etched in a desired pattern.

The twenty-third aspect of the present invention provides a method fordesigning an optically semitransmissive film which is formed on atranslucent substrate and which transmits a portion of light having adesired wavelength λ, the method comprising:

providing a phase-difference reduction layer that has a refractive indexn and a thickness d that satisfy the expression 0<d≦λ/(2(n−1));

taking into account a discontinuous negative phase change generated inthe boundary between the phase-difference reduction layer and a mediumselected from the translucent substrate, another layer, or atmosphereadjacent to the phase-difference reduction layer when light has passedthrough the optically semitransmissive film; and

adjusting the thickness of the phase-difference reduction layer anddesigning an optically semitransmissive film so that the phasedifference of the phase-difference reduction layer is a desired phasedifference.

The twenty-fourth aspect of the present invention provides the methodfor designing an optically semitransmissive film according to thetwenty-third aspect wherein the desired phase difference of thephase-difference reduction layer is set so that the phase difference ofthe entire optically semitransmissive film is in a range of −30° to+30°.

The twenty-fifth aspect of the present invention provides the method fordesigning an optically semitransmissive film according to thetwenty-third or twenty-fourth aspect wherein the phase-differencereduction layer is selected from materials in which the extinctioncoefficient k and the refractive index n satisfy the expression k≧n.

Effect of the Invention

In accordance with the present invention, the optically semitransmissivefilm has a refractive index n and a thickness d that satisfy theexpression 0<d≦λ(2(n−1)), and at least one phase-difference reductionlayer is provided in which the value Δθ is less than the value Δθ₀calculated based on the difference in the optical distance, making itpossible to obtain an optically semitransmissive film that has a nearlyzero phase shift, has a desired transmissivity, and is relatively thin.

In accordance with the present invention, a photomask blank can beobtained that can [be used to] manufacture a novel phase-shift mask asdescribed in Patent Document 1, for example, by using a photomask blankprovided with the optically semitransmissive film.

In accordance with the present invention, a novel phase shift photomasksuch as that described in Patent Document 1, for example, can beobtained in actual practice by manufacturing a photomask using theabove-described photomask blank.

BEST MODE FOR CARRYING OUT THE INVENTION

The optically semitransmissive film according to the present embodimentis formed on a translucent substrate, transmits a portion of lighthaving a desired wavelength λ, has a function for shifting the phase ofthe transmitted light by a prescribed amount, and is used in aphase-shift mask and a phase-shift mask blank, which is the material ofthe phase-shift mask. The optically semitransmissive film comprises aphase-difference reduction layer that has the functions described below.Following is a description of the optically semitransmissive filmaccording to the present invention, with focus placed on thephase-difference reduction layer.

First, the phase-difference reduction layer has one or more layershaving a refractive index n and a thickness d that satisfy theexpression 0<d≦λ(2(n−1)). The value Δθ (hereinafter referred to as phasedifference Δθ (units: degrees)) obtained by subtracting the phase oflight (hereinafter referred to as layer-referenced light) in the absenceof a layer from the phase of light (hereinafter referred to aslayer-transmitted light) transmitted through a layer is less than thevalue Δθ₀=(360/λ)×(n−1)×d (hereinafter referred to as the phasedifference Δθ₀ (units: degrees)) calculated based on the difference inthe optical distance between the layer-transmitted light and thelayer-referenced light, i.e., less than the value calculated using Eq. 1described above. The thickness d is made to be equal to or less than thethickness indicated in Eq. 2 described above, in which the conventionalphase difference is 180°. In this film-thickness region, a discontinuousphase difference produced when light passes through the boundary of twomedia is used in the present embodiment in order to obtain a phasedifference that is still less than the phase difference Δθ₀ calculatedin Eq. 1. This discontinuous phase change occurs when at least one ofthe two media is not transparent with respect to the transmitted light.

FIG. 1 is a diagram schematically showing an example of the phase changeof light that advances through two media. FIG. 1A shows a case in whichthe media 1 and 2 are both transparent, and FIG. 1B shows a case inwhich the medium 1 is transparent and the medium 2 is opaque. When themedia 1 and 2 are both transparent, the phase change is continuouswithin the two media and at the boundary between the two media, as shownin FIG. 1A. When the medium 2 is opaque, a discontinuous phasedifference is generated at the boundary between the media, as shown inFIG. 1B. The case in which the phase is delayed due to a discontinuousphase change is referred to as a negative phase change, as shown in FIG.1B.

The reason that the above-described discontinuous phase change occurs isdescribed in detail below.

Considered first is the process in which light is reflected at or passesthrough the boundary between two medium, as shown in FIG. 2. The complexrefractive index of the medium 1 is expressed as N₁=n₁−ik₁; the complexrefractive index of the medium 2 is expressed as N₂=n₂−ik₂; theamplitude of the reflected light r at the boundary of the media 1 and 2is A_(r)>0, and the phase is φ_(r); and the amplitude of the transmittedlight t at the boundary of the media 1 and 2 is A_(t)>0, and the phaseis φ_(t). In the above expressions, the refractive index of the medium 1is n₁, the extinction coefficient of the medium is k₁, the refractiveindex of the medium 2 is n₂, and the extinction coefficient of themedium is k₃ (*2). When the light that enters from the medium 1 into themedium 2 at the boundary 1-2 has an amplitude of 1 and a phase of 0, thefollowing expressions hold true for vertical incidence, based on Fresnellaws.r=A _(r)exp(−iφ _(r))=A _(r)(cos φ_(r) −i sinφ_(r))−(N1−N2)/(N1+N2)  (Eq. 4)t=A _(t)exp(−iφ _(t))=A _(t)(cos φ_(t) −i sin φ_(t))=2N1/(N1+N2)  (Eq.5)

When the media 1 and 2 are both transparent media, i.e., when k₁=k₂=0,the reflected light r and transmitted light t are real numbers.Therefore, the following hold true based on Eqs. 4 and 5.sin φ_(r)=sin φ_(t)=0A _(r) cos φ_(r)=(n ₁ −n ₂)(n ₁ +n ₂)→φ_(r)=π, π(n ₁ <n ₂)0(n ₁ >n ₂)A _(t) cos φ_(t)=2n ₁/(n ₁ +n ₂)>0→φ_(t)=0Since the phases of the incident light and the transmitted light are thesame, a discontinuous phase change does not occur at the boundary whenlight passes through the boundary of the media 1 and 2. However, thereflected light r and the transmitted light t are complex numbers whenone of the two media 1 and 2 is an absorbing medium. Therefore, sinφ_(t) does not become 0, and a discontinuous phase change occurs at theboundary when light passes through the boundary of the media 1 and 2.

FIG. 3A shows a case in which the refractive index n₁ and extinctioncoefficient k₁ of the medium 1 are set t be 1 and 0 (air or vacuum),respectively, and, based on Eq. 2, the phase change φ_(t) of the lightthat passes from the medium 1 to the medium 2 is calculated and thecontour lines are plotted when the refractive index n₂ and theextinction coefficient k₂ of the medium 2 are varied (the horizontalaxis represents the refractive index n, and the vertical axis representsthe extinction coefficient k). Based on the same theory, FIG. 3B showsthe phase change φ_(t)′ when light passes from the medium 2 to themedium 1 (the horizontal axis represents the refractive index n, and thevertical axis represents the extinction coefficient k).

It is apparent from FIGS. 3A and 3B that the phase of light changes in adiscontinuous manner by being reflected or transmitted at the boundarybetween the two media. Specifically,

-   -   a negative phase change occurs in the region in which the        extinction coefficient k is k>0 when light passes from the        medium 1 to the medium 2,    -   a positive phase change occurs in the region in which the        extinction coefficient k is k>0 when light passes from the        medium 2 to the medium 1,    -   the change of phase at the boundary of the media 1 and 2 depends        not only on the refractive index n, but also on the extinction        coefficient k, and    -   the medium having a smaller refractive index n and a larger        extinction coefficient k causes a negative phase change that        increases negativity at the boundary when light passes from the        medium 1 to the medium 2.

In the present embodiment, when the optically semitransmissive film hasa single phase-difference reduction layer, the medium 1 can be made tocorrespond to a translucent substrate, and the medium 2 can be made tocorrespond to a phase-difference reduction layer.

With any material that constitutes a phase shift region that correspondsto the medium 2 in a conventional half-tone phase-shift mask, theextinction coefficient k at the exposure wavelength is substantiallylower than the refractive index n and is close to 0. This results in alow value for the negative phase change at the boundary with thetranslucent substrate that corresponds to the medium 1, or a lowpositive phase change at the boundary with the atmosphere (air), and acontinuous phase change such as that shown in FIG. 1A can be presumed tooccur. On the other hand, with an absorbing material in which theextinction coefficient k is considerable, at about n≦k, relative to therefractive index n, consideration must be given both to thediscontinuous phase change at the boundary and to the conventional phasedifference shown in Eqs. 1 and 3.

The phase φ_(t) of the layer-transmitted light t of the phase-differencereduction layer, and the phase φ_(t)′ of the layer-referenced light t′can be calculated when a phase-difference reduction layer having acomplex refractive index N=n−ik is layered to a thickness d on a portionof a translucent substrate, and light having a wavelength λ is madevertically incident from the reverse (the surface with nophase-difference reduction layer) side of the substrate, as shown inFIG. 4. The transmitted light t and t′ are expressed in the followingmanner when the complex refractive index of the atmosphere is set to be1−0i, the complex refractive index of the translucent substrate is setto be N_(s)=n_(s)−0i, and consideration is given to multiple reflectionsof the light in the medium in addition to the aforementioned Fresnellaws.t=(t _(s) ×t _(l)×exp(−iδ))/(1+r _(s) ×r _(l)×exp(−2iδ)  (Eq. 6)t′=2n _(s)/(n _(s)+1)exp(−iδ′)|  (Eq. 7)In the equations,t _(s)=2n _(s)/(n _(s) +N)t _(l)=2N/(N+1)r _(s)=(n _(s) −N)/(n _(s) +N)r _(l)=(n _(s)−1)/(n _(s)+1)δ=(2π/λ)Nd, andδ′=(2π/λ)d|.As used herein, the term “phase θ_(t)′” corresponds to δ′. Also, thephase θ_(t) is calculated by rewriting Eq. 6 as t=Aexp (−ix) and usingx=θ_(t) (where A is a real arithmetic constant).

FIG. 5 is a graph obtained by plotting, as a function of the phasedifference Δθ_(t)=(θ_(t)−θ_(t)′) for the cases in which k=0.6 (dottedline), k=2.0 (alternate long and short dash line), and k=2.5 (dashedline), wherein n_(s), n, and λ in FIG. 4 are n_(s)=1.56, n=2.0, andλ=193.4 (nm). The graph also shows (solid line) the relationship betweenthe thickness d and the phase difference Δθn obtained from Eq. 1 forcalculating the phase difference in a conventional single-layerphase-shift mask.

It is apparent from FIG. 5 that the relationship between the thicknessand the phase difference of a film in which k=0.6, i.e., the extinctioncoefficient k is less than the refractive index n, is similar to therelationship in the Eq. 1. The case of a film in which k=2.0 and k=2.5,i.e., an absorbing film, considerably diverges from the relationship inEq. 1, and the phase difference is reduced. A film thickness region isproduced in which Δθ<0 when n<k=2.5. This is due to the effect of thenegative phase change produced at the boundary between the translucentsubstrate and the phase-difference reduction layer due to the fact thatthe extinction coefficient k is greater than the refractive index n.

Thus, the phase-difference reduction layer in the opticallysemitransmissive film of present embodiment comprises an absorbing film,i.e., an absorbing material in which the extinction coefficient k isrelatively large. Therefore, a discontinuous negative phase changeoccurs at the boundary with the medium selected from the translucentsubstrate, another layer, or atmosphere adjacent to the phase-differencereduction layer, and the negative phase change sufficiently small to beignored in conventional practice can be increased in the boundarybetween the substrate and the optically semitransmissive film. As aresult, the phase difference Δθ can be made less than the phasedifference Δθ₀ calculated using Eq. 1, and it is possible to obtain aphase-difference reduction layer that has a phase difference Δθ in thevicinity of 0°, e.g., a phase difference Δθ that is 0±30°, while havinga limited thickness (in practice, 1 nm or more, for example). Also, ifthe negative phase change is increased, an optically semitransmissivefilm (phase-difference reduction layer) can be formed in which the phasedifference Δθ is exactly 0° or is a negative value in a film thicknessregion that is less than the thickness at which the phase difference Δθ₀is Δθ=180° as calculated using Eq. 2. In the present embodiment, thephrase “phase difference Δθ₀ having a negative value” refers a case inwhich the phase difference is a negative value when expressed in therange of ±180°. In such a case, a phase difference of 360 degrees isexpressed as 0 degrees.

In the present embodiment, the thickness of the phase-differencereduction layer is not adjusted based on Eq. 1, but rather therelationship between the phase difference and the thickness as shown inFIG. 5, for example, is calculated in advance using Eqs. 6 and 7; andthe thickness is set so that a desired phase difference can be obtained.In particular, a case can be considered in which the thickness d and thephase difference Δθ are varied so that k=2.0 and k=2.5, and a materialis used for which the relationship is such that the phase differencesequentially changes from Δθ<0 to Δθ=0 to Δθ>0 with increased filmthickness from the point at which Δθ=0 at d=0, as shown in FIG. 5. Inthis case, a zero point through which the phase change passes whenchanging from negative to positive can be selected as the phasedifference, making it possible to obtain a phase-difference reductionlayer in which the phase difference is exactly zero. The transmissivitycan be adjusted by selecting a thickness that allows the phasedifference to be adjusted in the ranges of 0±30°, 0±10°, 0±5°, and thelike, based on the balance between the allowable phase difference andthe transmissivity required in the phase-difference reduction layer.

The transmissivity and phase difference of the opticallysemitransmissive film are suitably determined by design in accordancewith the intended application of the optically semitransmissive film. Inother words, a film material having a desired extinction coefficient kand a desired refractive index n can be selected so that the filmthickness satisfies the required value of the transmissivity and therequired value of the phase difference in accordance with the intendedapplication. Foreseeing variations in the transmissivity and phasedifference that accompany washing, heat treatment, and other treatmentsof the optically semitransmissive film, the film may be designed so thatthe transmissivity and phase difference are different than thetransmissivity and phase difference that are originally required whenthe optically semitransmissive film is formed. The balance between thetransmissivity variation and reflectivity variation that accompanywashing, heat treatment, and other treatments of the opticallysemitransmissive film can be used to ensure that the transmissivitysubstantially does not vary even when washing, heat treatment, and othertreatments are carried out, or to control the variation in thetransmissivity caused by the aforementioned washing, heat treatment, andother treatments.

As the difference between the phase difference Δθ and the phasedifference Δθ₀ increases above 10 degrees or more, for example, therelationship between the film thickness and the phase difference Δθbegins to form a broad curve, and the transmissivity, which variesdepending on the film thickness, can be more easily adjusted. When thephase-difference reduction layer is a single-layer opticallysemitransmissive film, the phase difference and transmissivity of theoptically semitransmissive film can be adjusted by adjusting the phasedifference and transmissivity of the phase-difference reduction layer.

The optically semitransmissive film is not limited to being providedwith a single phase-difference reduction layer, but may be provided withtwo or more phase-difference reduction layers (same or differentcharacteristics), or may be multilayered in combination with otherlayers.

An example of a multilayered-optically semitransmissive film is anoptically semitransmissive film having a multilayered structurecomprising at least one phase-difference reduction layer in which thephase difference is negative and at least one layer in which the phasedifference is positive, and in which the phase difference of the entireoptically semitransmissive film is a desired phase difference whereinthe negative phase difference and the positive phase difference cancelout each other.

When, for example, the reflectivity of the exposure light is high onlyin the phase-difference reduction layer, an antireflection layer can bedisposed on the phase-difference reduction layer. Specifically, thephase difference is positive because an antireflection film cannotordinarily sufficiently demonstrate [antireflective] function if thematerial does not have a low extinction coefficient. Therefore, thephase difference in the phase-difference reduction layer is negative,and the phase difference of the antireflective layer having a positivephase difference cancel out each other, whereby the phase difference ofthe entire optically semitransmissive film can be adjusted to near zero.

A relatively thin optically semitransmissive film can be obtained inaccordance with the present embodiment. The phase-difference reductionlayer preferably has a thickness selected from a range of 1 to 50 nm,and more preferably 1 to 30 nm. The thickness of the entire opticallysemitransmissive film is also preferably 1 to 50 nm, and more preferably1 to 30 nm. Thus, by making the film relatively thin, a pattern withvery small dimensions can be processed when a photomask is used or whenother processes are carried out in which a pattern must be formed on anoptically semitransmissive film.

Following is a description of the photomask blank and the photomask ofthe present embodiment.

The photomask blank of the present embodiment may, for example, be usedfor obtaining the photomask described in Patent Document 1.

The photomask 1 described in Patent Document 1 is one in which a phasedifference of 180° is provided to the aperture 3 disposed in thelight-blocking portion (optically semitransmissive area) by cutting atranslucent substrate 4 as a phase shifter, as shown in FIG. 6, and inwhich the phase difference has been adjusted to 0±30° and thetransmissivity has been adjusted to 15% or less by using the opticallysemitransmissive film 22 (FIG. 7) of the present embodiment as thelight-blocking portion 2. Therefore, the photomask 10 of the presentembodiment has a structure in which the annular diaphragm 22 is disposedon the translucent substrate 4, as shown in FIG. 7.

The method shown in FIG. 8 is an example of a method for manufacturingthe photomask.

Specifically, a resist film 11 is formed on a photomask blank 10, and apattern is drawn on the resist film 11 using an electron beam or laser(FIG. 8A). Next, the resist film 11 is developed, the opticallysemitransmissive film 22 is etched, and the translucent substrate 4 isetched down to a depth that produces a phase difference of 180° (FIG.8B). The resist film 11 is subsequently peeled away (FIG. 8C). Next, aresist film 12 is coated over the entire surface, and a pattern is drawnusing electron beam lithography (FIG. 8D). The resist film 12 issubsequently developed, and the optically semitransmissive film 22 isetched (FIG. 8E). Lastly, the resist film 12 is peeled away (FIG. 8F) toobtain a photomask 1 having a light-blocking portion 2 (opticallysemitransmissive area).

Also, the photomask 1 of the present embodiment can be used as aphotomask having a light-blocking band. The light-blocking band is alight-blocking film composed of a chromium-based material, for example,formed on at least the peripheral portion of the pattern region, and isprovided to prevent the occurrence of transfer pattern defects producedby transmitted light in the peripheral portion of the pattern regionduring photomask use.

FIG. 9 is a diagram showing an example of the steps for manufacturingthe photomask having a light-blocking band.

First, a light-blocking film 13 is formed on the photomask blank 10shown in FIG. 7, a resist film 11 is formed on the light-blocking film,and a pattern is drawn using an electron beam or a laser (FIG. 9A).Next, the resist film 11 is developed, the light-blocking film 13 isetched, the optically semitransmissive film 22 is etched, and thetranslucent substrate 4 is etched down to a depth that produces a phasedifference of 180° (FIG. 9B). The resist film 11 is subsequently peeledaway (FIG. 9C). Next, a resist film 12 is coated over the entire surface(FIG. 9D), and a pattern is drawn using electron beam lithography. Theresist film 12 is subsequently developed, the light-blocking film 13 isetched, and the optically semitransmissive film 22 is etched (FIG. 9E).Next, the resist film 12 is removed (FIG. 9F), a resist film 14 is againcoated over the entire surface (FIG. 9G), a resist pattern is formed toleave only a region in which the light-blocking band is formed (FIG. 9h), the chromium in the transfer region I is thereafter etched (FIG. 9i), and the remaining resist film 14 is peeled away (FIG. 9 h) to obtaina photomask 15 having a light-blocking band 33 (FIG. 9 j).

No limitations are imposed by the pattern arrangement on the photomask 1and the manufacturing steps described above. In the manufacturing steps,for example, rather than etching down the translucent substrate 4 andthereafter forming a light-blocking portion 2 composed of an opticallysemitransmissive film 22 as described above, the light-blocking portion2 composed of the optically semitransmissive film 22 may be formed andthe translucent substrate 4 etched thereafter.

Also, the phase shifter formed in the aperture 3 may be a film thatcauses phase shifting. In such a case, the photomask blank 10 would beone in which a phase-shifting film is disposed on the opticallysemitransmissive film 22 or between the translucent substrate 4 and theoptically semitransmissive film 22.

The optically semitransmissive film 22 may comprise a single-layer filmor a film having two or more layers. In the case of two or more layers,at least one of the layers can be a phase-difference reduction layerthat produces a negative phase change at the boundary. When thereflectivity of the exposure light is high with only a phase-differencereduction layer, an antireflective layer can be disposed on thephase-difference reduction layer. In other words, the phase differenceis usually positive because the antireflective film is incapable ofadequately demonstrating an antireflective function if it is not made ofa material having a low extinction coefficient. Therefore, the phasedifference in the phase-difference reduction layer is set to be negativeso as to cancel out the phase difference of the antireflective film,which has a positive phase difference, whereby an antireflectivefunction can be provided and the phase difference of the entireoptically semitransmissive film can be adjusted to near zero. Theantireflective film may be disposed on the phase-difference reductionlayer (the side on which the surface of the photomask blank is present)when lower surface reflectivity is desired in the photomask blank. Theantireflective film may be disposed below the phase-difference reductionlayer (between the translucent substrate and the phase-differencereduction layer) when lower reflectivity is desired in the reversesurface, and the antireflective film may be disposed on both sides ofthe phase-difference reduction layer when lower reflectivity is desiredin both the surface and the reverse side. The reflectivity is adjustedby designing the film so as to obtain a reflectivity that is equal to orless than the allowable value at the desired wavelength in the desiredsurface within a range in which adjustment can be made withconsideration given to the phase difference and transmissivity in thecase of a single layer or multiple layers.

The optically semitransmissive film 22 may be a film that has beenheat-treated at about 150 to 500° C., for example, or more preferablyabout 250 to 500° C.; treated by UV or laser radiation; or treated inanother manner in order to prevent variability in the characteristicsdue heat processing or other processing after film formation, or toprevent variability in the optical properties caused by washing oranother chemical treatment.

When the optically semitransmissive film 22 of the present embodiment isused as a photomask blank 10, the optical characteristics(transmissivity, reflectivity, and phase difference) must be matched tothe exposure wavelength of the exposure device in which the photomask 1is to be used. The exposure wavelength of the photomask 1 is preferablyone in which consideration is given to exposure wavelengths in the rangeof 150 nm to 250 nm, which includes KrF excimer laser light (wavelength:248 nm), ArF excimer laser light (wavelength: 193 nm), and F₂ excimerlaser light (wavelength: 157 nm). Particularly preferred is an exposurewavelength in which consideration is given to exposure wavelengths inthe range of 150 nm to 200 nm, which includes ArF excimer laser light(wavelength: 193 nm) and F₂ excimer laser light (wavelength: 157 nm).These are next-generation exposure wavelengths. Also, the reflectivityof the surface (the film surface side) of the photomask blank ispreferably kept to 30% or less at the exposure wavelength. Thereflectivity is also preferably kept to 30% or less on the reverse sideof the photomask blank. The reflectivity is preferably kept to about 40%or less at the scanning wavelength of the photomask 1 and photomaskblank 10. Examples of scanning wavelengths include 257 nm, 266 nm, 364nm, 488 nm, and 633 nm.

Other factors must also be considered, such as resistance to the washingfluid, pattern workability, and other characteristics of the opticallysemitransmissive film 22.

EXAMPLES

The present invention is described in further detail below using thephotomask blank and photomask as examples and comparative examples.

Example 1

An optically semitransmissive film provided with a singlephase-difference reduction layer composed of chromium nitride was formedon a translucent substrate composed of synthetic quartz glass to obtaina photomask blank having the structure shown in FIG. 7. Specifically, anoptically semitransmissive film was formed by sputtering a chromiumtarget onto a translucent substrate in an atmosphere of argon andnitrogen.

The table in FIG. 10 shows the transmissivity and phase difference of anoptically semitransmissive film at an ArF excimer laser wavelength of193 nm, which is the exposure wavelength maintained when a photomask isused. The transmissivity was measured using a spectrophotometer, and thephase difference was measured using a phase difference measuring device(MPM-193, manufactured by Lasertec). In the present example, thetransmissivity was kept within a range of 5 to 15%.

In the present example, the transmissivity was kept within a range of 5to 15% while keeping the phase difference near zero by using arelatively thin film.

Example 2

An optically semitransmissive film provided with a singlephase-difference reduction layer composed of tantalum hafnium was formedon a translucent substrate composed of synthetic quartz glass to obtainthe photomask blank shown in FIG. 7. Specifically, an opticallysemitransmissive film was formed by sputtering a tantalum hafnium(Ta:Hf=8:2) target onto a translucent substrate in an argon atmosphere.

The table in FIG. 10 shows the results of measuring the transmissivityand phase difference of an optically semitransmissive film at an ArFexcimer laser wavelength of 193 nm, which is the exposure wavelengthmaintained when a photomask is used. The same measuring means as used inexample 1 were employed. In the present example, the transmissivity waskept within a range of 5 to 15%.

In the present example, the transmissivity was kept within a range of 5to 15% while keeping the phase difference near zero by using arelatively thin film.

Example 3

An optically semitransmissive film provided with a singlephase-difference reduction layer composed of silicon was formed on atranslucent substrate composed of synthetic quartz glass to obtain aphotomask blank shown in FIG. 7. Specifically, an opticallysemitransmissive film was formed by sputtering a silicon target onto atranslucent substrate in an argon atmosphere.

The table in FIG. 10 shows the results of measuring the transmissivityand phase difference of an optically semitransmissive film at an ArFexcimer laser wavelength of 193 nm of, which is the exposure wavelengthmaintained when a photomask is used. The same measuring means as used inexample 1 were employed. In the present example, the transmissivity waskept within a range of 5 to 15%.

In the present example, the transmissivity was kept within a range of 5to 15% while keeping the phase difference near zero by using arelatively thin film.

Comparative Examples 1 and 2

An optically semitransmissive film having a MoSin film as the opticallysemitransmissive portion was formed on a conventional halftonephase-shift mask on a translucent substrate composed of synthetic quartzglass to obtain the photomask blank shown in FIG. 7. Specifically, anoptically semitransmissive film was formed by sputtering a mixed targetcomposed of Mo and Si (Mo:Si=1:9 atomic ratio) in an atmosphere composedof argon and nitrogen.

A MoSin [film] does not have a thickness region in which the phasedifference is 0° or less in a region below the minimal thickness atwhich the phase difference is 180°. Therefore, in comparative example 1,the phase difference was kept very small in a region of relatively smallfilm thicknesses, but the transmissivity was still excessively high. Incomparative example 2, the thickness was adjusted to give the phasedifference a zero limit. However, the optically semitransmissive portionwas very thick at 100 nm or more, and the film was not suitable formicropatterning.

Examples 4 to 6

The photomask blanks of examples 1 to 3 were used to form photomasks inaccordance with the steps shown in FIG. 8 described above.

The optically semitransmissive film Cr—N of example 1 was dry etchedusing Cl₂+O₂ as etching gas, the TaHf of example 2 was dry etched usingCl₂ gas, and the Si of example 3 was dry etched using CF₄ gas. Thetranslucent substrate was dry etched using CF₄+O₂ as etching gas. Thetranslucent substrate had sufficient resistance to the etching of theoptically semitransmissive film.

In the photomasks of examples 4 to 6, which correspond to examples 1 to3, a good pattern shape was obtained with low dimensional error inmicropatterning.

Examples 7 and 8

The present examples involve manufacturing photomasks having alight-blocking band by using the photomask blanks of examples 2 and 3.

The photomasks were formed in accordance with the steps in FIG. 9 byusing the photomask blanks of examples 2 and 3. A chromium-based filmwas used as the light-blocking film for the light-blocking band, and thefilm was dry etched using Cl₄+O₂ as etching gas. The etching of examples2 and 3 was carried out in the same manner as in examples 5 and 6.Selective etching was possible because the optically semitransmissivefilm and light-blocking film both had good etching selectivity, and agood pattern shape was obtained with low dimensional error by etchingthe optically semitransmissive film using the light-blocking film as amask.

The same chromium material as that of the light-blocking material forthe light-blocking band was used as the optically semitransmissive filmmaterial, and it was therefore difficult to selectively etch thephotomask blank of example 1.

Examples 9 to 11

A phase-difference reduction layer 16 composed of molybdenum andsilicon, and an antireflective layer 17 composed of molybdenum, silicon,and nitrogen were layered on a translucent substrate 4 composed ofsynthetic quartz glass to form an optically semitransmissive film 22, asshown in FIG. 11, and a photomask blank 18 was obtained. Specifically, aphase-difference reduction layer 16 was formed by sputtering a targetcomposed of molybdenum and silicon (Mo:Si=1:9 atomic ratio) onto atranslucent substrate 4 in an atmosphere of argon or another inert gas.Molybdenum and silicon (Mo:Si=1:9 atomic ratio) were subsequentlysputtered in an atmosphere containing argon and nitrogen to form anantireflective layer 17 composed of MoSiN, whereby a two-layer opticallysemitransmissive film 22 was formed.

The table in FIG. 12 shows the film characteristics of the examples 9 to11. The refractive index, extinction coefficient, transmissivity,reflectivity, and phase difference are the values observed at an ArFexcimer laser wavelength of 193 nm, which is the exposure wavelengthmaintained when a photomask is used. The transmissivity and reflectivitywere measured using a spectrophotometer, and the phase difference wasmeasured using a phase difference measuring device (MPM-193,manufactured by Lasertec). In the present example, the transmissivitywas kept within a range of 5 to 15%.

FIG. 13 shows the relationship between the film thickness and the phasedifference in the material of the antireflective layer used in examples9 to 11. It is apparent from the diagram that the phase difference ofthe optically semitransmissive film can be adjusted to be a negativevalue by layering an antireflective layer on the phase-differencereduction layer.

FIG. 14 is a diagram showing the reflection spectrum of the surface ofthe photomask blank used in examples 9 to 11.

In examples 9 to 11, the transmissivity was kept within a range of 5 to15% while keeping the phase difference near zero by using a relativelythin film, as shown in FIG. 12. The reflectivity at the exposurewavelength was a relatively low value of 30% or less, and in example 11,the reflectivity was kept to 40% or less at 266 nm, which is thephotomask scanning wavelength, as shown in FIG. 14.

An MoSi-based material was used in the optically semitransmissive filmin examples 9 to 11. Therefore, when a photomask with a light-blockingband was manufactured in the same manner as in examples 7 and 8, theetching selectivity of the optically semitransmissive film and thelight-blocking film was high. Therefore, selective etching was possiblebecause the optically semitransmissive film and light-blocking film bothhad good etching selectivity, and a good pattern shape was obtained withlow dimensional error by etching the optically semitransmissive filmusing the light-blocking film as a mask.

In examples 9 to 11, an MoSiN antireflective layer was formed using atarget composed of molybdenum and silicon as a sputtering target, but asilicon target may be used to obtain a SiN antireflective layer. Oxygenmay furthermore be added to the film formation atmosphere to form anantireflective layer composed of MoSiO, SiO, MoSiON, SiON, and the like.In terms of resistance to alkali and acid washing, the molybdenumcontent of the antireflective layer and/or the phase-differencereduction layer is preferably less than 30%. Alternatively, themolybdenum and silicon ratio in the sputtering target is preferably setin a range of 0:100 to 30:70.

Reference Example 1

A photomask blank was fabricated using only a phase-difference reductionlayer, without the use of an antireflective film (see the table in FIG.12). As a result, the phase difference was kept to ±30° and thetransmissivity was kept within a range of 5 to 15%, but the reflectivitywas a very high value.

Comparative Example 3

A film composed of a material in which the refractive index n wasgreater than the extinction coefficient k was formed as thephase-difference reduction layer, and an antireflective layer was formedon the phase-difference reduction layer (see the table in FIG. 12). As aresult, the transmissivity was kept within a range of 5 to 15% and thereflectivity was reduced, but the film had a very considerable phasedifference.

The present invention is not limited to the examples described above.

Examples of a photomask blank and a photomask are described above, butthe present invention may be used in all applications of an opticallysemitransmissive film in which a near-zero phase difference is required.

In the examples described above, the transmissivity was designed to bekept within a range of 5 to 15%, but the design value of thetransmissivity can be varied in accordance with the design of thetransfer pattern when the photomask is being used. The design value ofthe transmissivity can be selected from a range that is 40% or less, forexample.

The material of the phase-difference reduction layer is furthermore notlimited to the aforementioned materials, and the material may be one inwhich a negative phase change is produced by the transmission and/orreflection of exposure light at the boundary with the atmosphere towhich the optically semitransmissive film is exposed, and/or at theboundary between the optically semitransmissive film and the translucentsubstrate. Specifically, the material of the phase-difference reductionlayer may contain one or more materials selected from Ta, Hf, Si, Cr,Ag, Au, Cu, Al, Mo, and other materials.

When, for example, the optically semitransmissive film is used as aphotomask blank, the material of the optically semitransmissive filmpreferably has a phase difference-reducing effect in the UV region, isresistant to alkali washing fluids and acid washing fluids, and hasexcellent patterning characteristics. Based on these points, it ispreferable to use a material containing silicon alone or both siliconand a metal (Mo, Ta, W, Cr, Zr, Hf, or another metal). From thestandpoint of photomask production, the material of the opticallysemitransmissive film preferably has a high etching selectivity ratio inrelation to the chromium light-blocking film as a light-blocking band.For example, the dimensional precision of the translucent substrateportion can be improved by including a material that prevents theoptically semitransmissive film from being etched during etching of atranslucent substrate.

With an optically semitransmissive film that has, e.g., a multilayeredstructure comprising a phase-difference reduction layer and anantireflective layer, configuring these layers from a material that canbe etched using the same etching medium is preferred from the standpointof simplifying the mask manufacturing steps when the film is patterned.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is diagram schematically showing the phase change of light thatadvances through two media;

FIG. 2 is a diagram showing transmitted light and reflected light at theboundary of two media;

FIG. 3 is a diagram showing the phase change of light that has passedbetween two media;

FIG. 4 is a diagram showing transmitted light and reflected light at theboundary of a translucent substrate and a phase-difference reductionlayer;

FIG. 5 is a diagram showing the relationship between the phasedifference and the thickness of the film;

FIG. 6 is a cross-sectional view showing an embodiment of the photomaskof the present invention;

FIG. 7 is a cross-sectional diagram showing the first embodiment of thephotomask blank of the present invention;

FIG. 8 is a diagram of the manufacturing steps showing the firstembodiment of the method for manufacturing a photomask of the presentinvention;

FIG. 9 is a diagram of the manufacturing steps showing anotherembodiment of the method for manufacturing a photomask of the presentinvention;

FIG. 10 is a table showing examples 1 to 3 of the present invention;

FIG. 11 is a cross-sectional view showing the photomask blank inexamples 9 to 11 of the present invention;

FIG. 12 is a table showing examples 9 to 11 of the present invention;

FIG. 13 is a diagram showing the relationship between the film thicknessand the phase difference of the photomask blank in examples 9 to 11 ofthe present invention; and

FIG. 14 is a reflection spectrum of the photomask blank in examples 9 to11 of the present invention.

KEY

-   1 photomask-   2 light-blocking portion-   3 aperture-   4 translucent substrate-   10 photomask blank-   11, 12, 14 resist films-   13 light-blocking film-   15 photomask having a light-blocking band-   16 phase-difference reduction layer-   17 antireflection layer-   18 photomask blank-   22 optically semitransmissive film-   33 light-blocking film

1. A photomask blank comprising: a transparent substrate and a film containing at least two layers formed on the transparent substrate, the two layers being a phase-difference reduction layer with a phase difference Δθ₁ being negative and a layer with a phase difference Δθ₂ being positive, wherein the phase difference Δθ₁ is a phase difference between a light transmitted through the phase-difference reduction layer and a light transmitted through an air distance equal to a thickness of the phase-difference reduction layer, wherein the phase difference Δθ₂ is a phase difference between a light transmitted through the layer with the phase difference Δθ₂ being positive and a light transmitted through an air distance equal to a thickness of the layer with the phase difference Δθ₂ being positive, and wherein the phase difference Δθ₁ and the phase difference Δθ₂ are canceled out, and a phase difference of the film containing at least two layers is from −30° to +30°.
 2. The photomask blank of claim 1, wherein the phase-difference reduction layer is comprised of a material containing one or more materials selected from the group consisting of Ta, Hf, Si, Cr, Ag, Au, Cu, Al, and Mo.
 3. The photomask blank of claim 2, wherein the phase-difference reduction layer is comprised a material containing Mo, and a content of Mo in the phase-difference reduction layer is less than 30%.
 4. The photomask blank of claim 1, wherein the phase-difference reduction layer is comprised of a material containing MoSi.
 5. The photomask blank of claim 1, wherein the phase-difference reduction layer is formed using a sputtering target in which a ratio of Mo to Si is from 0:100 to 30:70.
 6. The photomask blank of claim 1, wherein the phase-difference reduction layer is comprised of a material in which a refractive index n1 and an extinction coefficient k1 satisfy an expression k1≧n1.
 7. The photomask blank of claim 1, wherein an extinction coefficient k of the phase-difference reduction layer is 1.5 or more.
 8. The photomask blank of claim 1, wherein an extinction coefficient k of the phase-difference reduction layer is 2.0 or more.
 9. The photomask blank of claim 1, wherein an extinction coefficient k of the phase-difference reduction layer is 2.5 or more.
 10. The photomask blank of claim 1, wherein the layer with phase difference Δθ₂ being positive is comprised of a material containing one or more materials selected from the group consisting of MoSiN, MoSiO, MoSiON, SiN, SiO, and SiON.
 11. The photomask blank of claim 10, wherein the phase-difference reduction layer is comprised of a material containing Mo and a content of Mo in the layer with phase difference Δθ₂ being positive is less than 30%.
 12. The photomask blank of claim 1, wherein the layer with phase difference Δθ₂ being positive is formed using a sputtering target in which a ratio of Mo to Si is from 0:100 to 30:70.
 13. The photomask blank of claim 1, wherein the layer with phase difference Δθ₂ being positive is an antireflective layer.
 14. The photomask blank of claim 13, wherein the antireflective layer is disposed on the phase-difference reduction layer.
 15. The photomask blank of claim 1, wherein the layer with phase difference Δθ₂ being positive is comprised of a material of which a refractive index n2 and an extinction coefficient k2 satisfy an expression k2<n2.
 16. The photomask blank of claim 1, wherein a thickness of the film is from 1 to 50 nm.
 17. The photomask blank of claim 1, wherein a thickness of the film is from 1 to 30 nm.
 18. A photomask manufactured by using the photomask blank of claim
 1. 